a) Field of the Invention
The present invention relates to a vari-focal lens system to be used in optical instruments.
b) Description of the Prior Art
In recent years where optical instruments such as video cameras and digital cameras are prevailing, zoom lens systems which are capable of changing focal lengths are adopted for these instruments at an increasing ratio. However, a zoom lens system constitutes an obstacle to configuring of video cameras compactly and digital cameras and manufacturing these instruments at low costs since 10 or more lens elements are required to compose a zoom lens system which uses a large number of lens elements. Further, a zoom lens system for silver salt cameras also constitutes an obstacle to reducing manufacturing cost thereof since it requires 10 or more lens elements when it is composed of a large number of lens elements.
Further, it is desirable for a camera such as a video camera which uses an electronic image pickup device to dispose a nearly telecentric lens system on the image side. For this reason, it is desirable to configure a lens unit which is to be disposed on the image side as a lens unit having a positive refractive power and dispose an aperture stop on the object side of this lens unit. Generally disposed on the image side is a vari-focal lens unit which has a positive refractive power (hereinafter referred to as a final lens unit).
This final lens unit has an imaging function and a strong refractive power as a whole, and is ordinarily composed of a large number of lens elements, or three to six lens elements.
A zoom lens system of this type requires tedious procedures for working lens elements and a lens barrel and is highly sensible of eccentricities, thereby requiring a long time for assembly and adjustment, and making it difficult to lower a manufacturing cost thereof. Since this zoom lens system is composed of a large number of lens elements, it produces large amounts of rays reflected by surfaces, ghost rays and flare spots which are peculiar to a vari-focal lens system.
Since a number of lens elements used in this imaging lens unit (the final lens unit) can be reduced only within a certain limit so long as only homogeneous lens elements are adopted for the lens unit, aspherical lens elements and/or gradient index lens elements are used for reducing the number of lens elements in this lens unit.
Since a radial type gradient index lens element has a high aberration correcting capability in particular, it is possible to reduce the number of lens elements, configure the lens unit more compact and lower a manufacturing cost thereof.
As conventional examples of zoom lens systems which use radial type gradient index lens elements, there are known lens systems disclosed by Japanese Patents Kokai Publication No. Hei 4-42113, Kokai Publication No.
Hei 7-159694, Kokai Publication No. Hei 2-56515 and Kokai Publication No. Hei 6-337347.
Out of the these conventional examples, the lens system disclosed by Japanese Patent Kokai Publication No.
Hei 4-42113 uses an imaging lens unit (final lens unit) which is composed of two radial type gradient index lens elements.
Further, the lens system disclosed by Japanese Patent Kokai Publication No. Hei 7-159694 uses an imaging lens unit (final lens unit) which is composed of three lens elements: a radial type gradient index lens element and two homogeneous lens elements.
Furthermore, the lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-56515 is composed of two radial type gradient index lens elements.
Moreover, the lens system disclosed by Japanese Patent Kokai Publication No. Hei 6-337347 is composed of three positive, negative and positive lens units and uses a gradient index lens element having a shape of a plane parallel plate in a final lens unit to correct chromatic aberration.
The lens system disclosed by Japanese Patent Kokai Publication No. Hei 4-42113 uses an imaging lens unit (final lens unit) which is composed of lens element in a number as small as two. This lens system is not preferable from viewpoints of workability and allowance since each of the two lens elements is a radial type gradient index lens element and has curvature on two surfaces thereof.
The lens system disclosed by Japanese Patent Kokai Publication No. Hei 7-159694 is not preferable from viewpoints of workability, allowance and manufacturing cost since the imaging lens unit (final lens unit) is composed of lens elements in a number as large as three, or two homogeneous lens elements and a radial type gradient index lens element, and the radial type gradient index lens element has curvature on two surfaces thereof.
Though the lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-56515 is composed of a small number of lens elements, this lens system is not preferable from viewpoints of workability, allowance and a manufacturing cost since the lens system uses a plurality of radial type gradient index lens elements each of which has curvature on two surfaces thereof.
Though the lens system disclosed by Japanese Patent Kokai Publication No. Hei 6-337347 uses a gradient index lens element which has two planar surfaces, this lens system can hardly allow a manufacturing cost thereof to be reduced, and produces large amounts of rays reflected by surfaces, ghost rays and flare spots since the imaging lens unit (final lens unit) is composed of a large number of lens elements, and requires assembly and adjustment.